Due to its physical properties polyester film is an excellent support for photographic film. However, it has been difficult to obtain good adhesion towards hydrophilic photographic emulsions because polyester, as is known, is generally very hydrophobic since it does not have active functional groups for the hydrophilic emulsion to attach to. There have been suggested several ways in the prior art to render the surface of a polyester film hydrophilic so that a photographic emulsion can be directly applied onto that surface. Such methods comprised chemical treatments such as etching or corona, flame or plasma treating the polyester film surface. Another approach for rendering hydrophilicity to the film surface was to apply a subbing layer onto the polyester film. Such a subbing layer would perform a double function, i.e. it would have good adhesion towards the polyester base film on the one side and at the same time good adhesion towards the photographic emulsion. Other methods known in the art first apply an auxiliary primer layer to the polyester film surface and thereafter a subbing layer on top of the auxiliary layer. The photographic layer is then applied onto the subbing layer. The purpose of such primer layers is to enhance the adhesion of the subbing layer towards the polyester film.
Miller, U.S. Pat. No. 4,701,403 describes an antistatic coating for polyester film which coating comprises a water soluble copolymer of the sodium salt of styrenesulfonic acid and a carboxy containing monomer which copolymer is crosslinked after drying with an aziridine component. Cho, U.S. Pat. No. 4,960,687 also describes a subbing layer containing a polymer having functionally attached carboxylic acid groups such as poly- (sodium styrenesulfo-nate-maleic-anhydride) and a polyfunctional aziridine crosslinking agent for such COOH-containing polymer, which crosslinking agent is dispersed in gelatin. Ochiai et at., U.S. Pat. 4,124,395 also suggests copolymers made from maleic acid anhydride and styrene sulfonic acid in combination with an addition polymerizable monomer which may be acrylic acid, itaconic acid or triazines. Optionally a crosslinking agent may be added to the subbing composition such as crosslinking agents having a hydroxy group.
All of these known subbing layers either employ hazardous materials such as aziridines or require the use of organic solvents, the use of which in production plants is subject to severe restrictions. It was therefore an object of the instant invention to suggest a subbing layer composition which neither employs hazardous material nor requires the use of organic solvents. While accomplishing this the subbed films should at the same time provide sufficient adhesion towards a subsequently applied photographic layer. It further would be advantageous if the subbed film exhibited antistatic properties.